Carbon-containing solids such as oil shale or tar sand are wide-spread in nature and the known deposits contain vast amounts of potential energy. All of the previously known methods for treating these solids make only oil as an economically transportable product. The potential energy of the solid which is not converted to oil ends up as a combustion gas (producer gas or retort gas), or as combustion heat, or as a non-volatile coke deposited on the processed solid.
Producer gas or retort gas can be transported economically for only short distances. Combustion heat must be either used at the site of combustion or else discarded. Non-volatile carbon on the treated solid is necessarily wasted when the solid is discarded.
The amount of carbonaceous residue remaining after pyrolysis of oil shales or tar sands varies with the material treated. The richest of Colorado shales leave about 20% of the original carbon in the treated (retorted) shale. Devonian shales of Kentucky normally leave more than 50% of the original carbon in the discarded shale. Coaly shales and sands exist which leave more than 80% of the original carbon on the waste solids from pyrolysis retorting. These last materials are not now considered potential resources at all since they yield little oil upon pyrolysis.
The treatment of carbon-containing solids entails costly mining and processing of much more inert material than the end products. It is highly desirable to obtain as high a yield as possible of high-grade transportable products in order to lower the unit product costs of mining and processing.